When performing mud gas logging at the surface of a drilling site, one method of analyzing the gas onsite is the combined use of a gas chromatograph and a mass spectrometer. From this analysis, indicators such as “wetness” (W), “balance” (B) and “character” (C) can be calculated. These indicators provide information about the maturity and nature of hydrocarbons comprising the source of the oil accumulation, compartmentalization of the reservoir being drilled, and oil quality, as well as information regarding production zones, lithology changes, history of a reservoir accumulation, or seal effectiveness.
However, surface mud gas logging has intrinsic limitations, such as contamination of the gas sample at the surface by air requiring the use of strict quality control procedures, or sampling contamination caused by fluids from previously drilled layers. Furthermore, such measurements are intrinsically inaccurate because the depth of the sampling location is inferred from gross measurements of mud transit time and average mud velocity.